Solar energy technologies, paired with energy conservation,
have the potential to meet a large portion of future US energy
needs

The United States faces serious energy shortages in the near
future. High energy consumption and the ever-increasing US
population will force residents to confront the critical problem
of dwindling domestic fossil energy supplies. With only 4.7% of
the world's population, the United States consumes approximately
25% of the total fossil fuel used each year throughout the world.
The United States now imports about one half of its oil (25% of
total fossil fuel) at an annual cost of approximately $65 billion
(USBC 1992a). Current US dependence on foreign oil has important
economic costs (Gibbons and Blair 1991) and portends future
negative effects on national security and the economy.

Domestic fossil fuel reserves are being rapidly depleted, and
it would be a major drain on the economy to import 100% of US
oil. Within a decade or two US residents will be forced to turn
to renewable energy for some of their energy needs. Proven US oil
reserves are projected to be exhausted in 10 to 15 years
depending on consumption patterns (DOE 1991a, Matare 1989,
Pimentel et al. 1994, Worldwatch Institute 1992), and natural gas
reserves are expected to last slightly longer. In contrast, coal
reserves have been projected to last approximately 100 years,
based on current use and available extraction processes (Matare
1989).

The US coal supply, however, could be used up in a much
shorter period than the projected 100 years, if one takes into
account predicted oil and gas depletion and concurrent population
growth (DOE 1991a, Matare 1989). The US population is projected
to double to more than one-half billion within the next 60 years
(USBC 19921). How rapidly the coal supply is depleted will depend
on energy consumption rates. The rapid depletion of US oil and
gas reserves is expected to necessitate increased use of coal. By
the year 2010, coal may constitute as much as 40% of total energy
use (DOE 1991a). Undoubtedly new technologies will be developed
that will make it possible to extract more oil and coal. However,
this extra extraction can only be achieved at greater energy and
economic costs. When the energy input needed to power these
methods approaches the amount of energy mined, extraction will no
longer be energy cost-effective (Hall et al. 1986).

Fossil fuel combustion, especially that based on oil and coal,
is the major contributor to increasing carbon dioxide
concentration in the atmosphere, thereby contributing to probable
global warming. This climate change is considered one of the most
serious environmental threats throughout the world because of its
potential impact on food production and processes vital to a
productive environment. Therefore, concerns about carbon dioxide
emissions may discourage widespread dependence on coal use and
encourage the development and use of renewable energy
technologies.

Even if the rate of increase of per capita fossil energy
consumption is slowed by conservation measures, rapid population
growth is expected to speed fossil energy depletion and intensify
global warming. Therefore, the projected availability of all
fossil energy reserves probably has been overstated.
Substantially reducing US use of fossil fuels through the
efficient use of energy and the adoption of solar energy
technologies extends the life of fossil fuel resources and could
provide the time needed to develop and improve renewable energy
technologies.

Renewable energy technologies will introduce new conflicts.
For example, a basic parameter controlling renewable energy
supplies is the availability of land. At present more than 99% of
the US and world food supply comes from the land (FAO 1991). In
addition, the harvest of forest resources is presently
insufficient to meet US needs and thus the United States imports
some of its forest products (USBC 1992a). With approximately 75%
of the total US land area exploited for agriculture and forestry,
there is relatively little land available for other uses, such as
biomass production and solar technologies. Population growth is
expected to further exacerbate the demands for land. Therefore,
future land conflicts could be intense.

In this article, we analyze the potential of various renewable
or solar energy technologies to supply the United States with its
future energy needs. Diverse renewable technologies are assessed
in terms of their land requirements, environmental benefits and
risks, economic costs, and a comparison of their advantages. In
addition, we make a projection of the amount of energy that could
be supplied by solar energy subject to the constraints of
maintaining the food and forest production required by society.
Although renewable energy technologies often cause fewer
environmental problems than fossil energy systems, they require
large amounts of land and therefore compete with agriculture,
forestry, and other essential land-use systems in the United
States.

Assessment of renewable energy technologies

Coal, oil, gas, nuclear, and other mined fuels currently
provide most of US energy needs. Renewable energy technologies
provide only 8% (Table 1).

At present, forest biomass energy, harvested from natural
forests, provides an estimated 3.6 quads (1.1 x 10 18 Joules) or
4.2% of the US energy supply (Table 1). Worldwide, and especially
in developing countries, biomass energy is more widely used than
in the United States. Only forest biomass will be included in
this US assessment, because forest is the most abundant biomass
resource and the most concentrated form of biomass. However, some
biomass proponents are suggesting the use of grasses, which on
productive soils can yield an average of 5 t · ha-1 yr-1 (Hall
et al. 1993, USDA 1992).

Although in the future most biomass probably will be used for
space and water heating, we have analyzed its conversion into
electricity in order to clarify the comparison with other
renewable technologies. An average of 3 tons of (dry) woody
biomass can be sustainably harvested per hectare per year with
small amounts of nutrient fertilizer inputs (Birdsey 1992). This
amount of woody biomass has a gross energy yield of 13.5 million
kcal (thermal). The net yield is, however, lower because
approximately 33 liters of diesel fuel oil per hectare is
expended for cutting and collecting wood and for transportation,
assuming an 80 kilometer roundtrip between the forest and the
plant. The economic benefits of biomass are maximized when
biomass can be used close to where it is harvested.

A city of 100,000 people using the biomass from a sustainable
forest (3 tons/ha) for fuel would require approximately 220,000
ha of forest area, based on an electrical demand of 1 billion kWh
(860 x 109 kcal = 1 kWh) per year (Table 2). Nearly 70% of the
heat energy produced from burning biomass is lost in the
conversion into electricity, similar to losses experienced in
coal fired plants. The area required is about the same as that
currently used by 100,000 people for food production, housing,
industry, and roadways (USDA 1992).

The energy input/output ratio of this system is calculated to
be 1:3 (Table 2). The cost of producing a kilowatt of electricity
from woody biomass ranges from 7˘ to 10˘ (Table 2), which is
competitive for electricity production that presently has a cost
ranging from 3˘ to 13˘ (Table 2; USBC 1992a ). Approximately 3
kcal of thermal energy is required to produce 1 kcal of
electricity. Biomass could supply the nation with 5 quads of its
total gross energy supply by the year 2050 with the use of at
least 75 million ha (an area larger than Texas, or approximately
8% of the 917 million ha in the United States) (Table 3).

However, several factors limit reliance on woody biomass.
Certainly, culturing fast-growing trees in a plantation system
located on prime land might increase yields of woody biomass.
However, this practice is unrealistic because prime land is
essential for food production. Furthermore, such intensely
managed systems require additional fossil fuel inputs for heavy
machinery, fertilizers, and pesticides, thereby diminishing the
net energy available. In addition, Hall et al. (1986) point out
that energy is not the highest priority use of trees.

If natural forests are managed for maximal biomass energy
production, loss of biodiversity can be expected. Also, the
conversion of natural forests into plantations increases soil
erosion and water runoff. Continuous soil erosion and degradation
would ultimately reduce the overall productivity of the land.
Despite serious limitations of plantations, biomass production
could be increased using agroforestry technologies designed to
protect soil quality and conserve biodiversity. In these systems,
the energy and economic costs would be significant and therefore
might limit the use of this strategy.

The burning of biomass is environmentally more polluting than
gas but less polluting than coal. Biomass combustion releases
more than 100 different chemical pollutants into the atmosphere
(Alfheim and Ramdahl 1986). Wood smoke is reported to contain
pollutants known to cause bronchitis, emphysema, and other
illnesses. These pollutants include up to 14 carcinogens, 4
cocarcinogens, 6 toxins that damage cilia, and additional
mucus-coagulating agents (Alfheim and Ramdahl 1986, DOE 1980). Of
special concern are the relatively high concentrations of
potentially carcinogenic polycyclic aromatic hydrocarbons (PAHs,
organic compounds such as benzo(a)pyrene) and particulates found
in biomass smoke (DOE 1980). Sulfur and nitrogen oxides, carbon
monoxide, and aldehydes also are released in small though
significant quantities and contribute to reduced air quality (DOE
1980). In electric generating plants, however, as much as 70% of
these air pollutants can be removed by installing the appropriate
air-pollution control devices in the combustion system.

Because of pollutants, several communities (including Aspen,
Colorado) have banned the burning of wood for heating homes. When
biomass is burned continuously in the home for heating, its
pollutants can be a threat to human health (Lipfert et al. 1988,
Smith 1987b).

When biomass in the form of harvested crop residues is used
for fuel, the soil is exposed to intense erosion by wind and
water (Pimentel et al. 1984). In addition to the serious
degradation of valuable agricultural land, the practice of
burning crop residues as a fuel removes essential nutrients from
the land and requires the application of costly fossil-based
fertilizers if yields are to be maintained. However, the soil
organic matter, soil biota, and water-holding capacity of the
soil cannot be replaced by applying fertilizers. Therefore, we
conclude that crop residues should not be removed from the land
for a fuel source (Pimentel 1992).

Biomass will continue to be a valuable renewable energy
resource in the future, but its expansion will be greatly
limited. Its use conflicts with the needs of agricultural and
forestry production and contributes to major environmental
problems.

Liquid fuels

Liquid fuels are indispensable to the US economy (DOE 1991a).
Petroleum, essential for the transportation sector as well as the
chemical industry, makes up approximately 42% of total US energy
consumption. At present, the United States imports about one half
of its petroleum and is projected to import nearly 100% within 10
to 15 years (DOE 1991a). Barring radically improved electric
battery technologies, a shift from petroleum to alternative
liquid and gaseous fuels will have to be made. The analysis in
this section is focused on the potential of three liquid fuels:
ethanol, methanol, and hydrogen.

Ethanol. A wide variety of starch and sugar crops, food
processing wastes, and woody materials (Lynd et al. 1991) have
been evaluated as raw materials for ethanol production. In the
United States, corn appears to be the most feasible biomass
feedstock in terms of availability and technology (Pimentel
1991).

The total fossil energy expended to produce 1 liter of ethanol
from corn is 10,200 kcal, but note that 1 liter of ethanol has an
energy value of only 5130 kcal. Thus, there is an energy
imbalance causing a net energy loss. Approximately 53% of the
total cost (55˘ per liter) of producing ethanol in a large,
modern plant is for the corn raw material (Pimentel 1991). The
total energy inputs for producing ethanol using corn can be
partially offset when the dried distillers grain produced is fed
to livestock. Although the feed value of the dried distillers
grain reduces the total energy inputs by 8 % to 24%, the energy
budget remains negative.

The major energy input in ethanol production, approximately
40% overall, is fuel needed to run the distillation process
(Pimentel 1991). This fossil energy input contributes to a
negative energy balance and atmospheric pollution. In the
production process, special membranes can separate the ethanol
from the so-called beer produced by fermentation. The most
promising systems rely on distillation to bring the ethanol
concentration up to 90%, and selective-membrane processes are
used to further raise the ethanol concentration to 99.5% (Maeda
and Kai 1991). The energy input for this upgrading is
approximately 1280 kcal/liter. In laboratory tests, the total
input for producing a liter of ethanol can potentially be reduced
from 10,200 to 6200 kcal by using membranes, but even then the
energy balance remains negative.

Any benefits from ethanol production, including the corn
by-products, are negated by the environmental pollution costs
incurred from ethanol production (Pimentel 1991). Intensive corn
production in the United States causes serious soil erosion and
also requires the further draw-down of groundwater resources.
Another environmental problem is caused by the large quantity of
stillage or effluent produced. During the fermentation process
approximately 13 liters of sewage effluent is produced and placed
in the sewage system for each liter of ethanol produced.

Although ethanol has been advertised as reducing air pollution
when mixed with gasoline or burned as the only fuel, there is no
reduction when the entire production system is considered.
Ethanol does release less carbon monoxide and sulfur oxides than
gasoline and diesel fuels. However, nitrogen oxides,
formaldehydes, other aldehydes, and alcohol--all serious air
pollutants-- are associated with the burning of ethanol as fuel
mixture with or without gasoline (Sillman and Samson 1990). Also,
the production and use of ethanol fuel contribute to the increase
in atmospheric carbon dioxide and to global warming, because
twice as much fossil energy is burned in ethanol production than
is produced as ethanol.

Ethanol produced from corn clearly is not a renewable energy
source. Its production adds to the depletion of agricultural
resources and raises ethical questions at a time when food
supplies must increase to meet the basic needs of the rapidly
growing world population.

Methanol. Methanol is another potential fuel for
internal combustion engines (Kohl 1990). Various raw materials
can be used for methanol production, including natural gas, coal,
wood, and municipal solid wastes. At present, the primary source
of methanol is natural gas. The major limitation in using biomass
for methanol production is the enormous quantities needed for a
plant with suitable economies of scale. A suitably large methanol
plant would require at least 1250 tons of dry biomass per day for
processing (ACTI 1983). More than 150,000 ha of forest would be
needed to supply one plant. Biomass generally is not available in
such enormous quantities from extensive forests and at acceptable
prices (ACTI 1983).

If methanol from biomass (33 quads) were used as a substitute
for oil in the United States, from 250 to 430 million ha of land
would be needed to supply the raw material. This land area is
greater than the 162 million ha of US cropland now in production
(USDA 1992). Although methanol production from biomass may be
impractical because of the enormous size of the conversion plants
(Kohl 1990), it is significantly more efficient than the ethanol
production system based on both energy output and economics (Kohl
1990).

Compared to gasoline and diesel fuel, both methanol and
ethanol reduce the amount of carbon monoxide and sulfur oxide
pollutants produced, however both contribute other major air
pollutants such as aldehydes and alcohol. Air pollutants from
these fuels worsen the tropospheric ozone problem because of the
emissions of nitrogen oxides from the richer mixtures used in the
combustion engines (Sillman and Samson 1990).

Hydrogen. Gaseous hydrogen, produced by the
electrolysis of water, is another alternative to petroleum fuels.
Using solar electric technologies for its production, hydrogen
has the potential to serve as a renewable gaseous and liquid fuel
for transportation vehicles. In addition hydrogen can be used as
an energy storage system for electrical solar energy
technologies, like photovoltaics (Winter and Nitsch 1988).

The material inputs for a hydrogen production facility are
primarily those needed to build a solar electric production
facility. The energy required to produce 1 billion kWh of
hydrogen is 1.3 billion kWh of electricity (Voigt 1984). If
current photovoltaics (Table 2) require 2700 ha/1 billion kWh,
then a total area of 3510 ha would be needed to supply the
equivalent of 1 billion kWh of hydrogen fuel. Based on US per
capita liquid fuel needs, a facility covering approximately 0.15
ha (16,300 ft2) would be needed to produce a year's requirement
of liquid hydrogen. In such a facility, the water requirement for
electrolytic production of 1 billion kWh/yr equivalent of
hydrogen is approximately 300 million liters/yr (Voigt 1984).

To consider hydrogen as a substitute for gasoline: 9.5 kg of
hydrogen produces energy equivalent to that produced by 25 kg of
gasoline. Storing 25 kg of gasoline requires a tank with a mass
of 17 kg, whereas the storage of 9.5 kg of hydrogen requires 55
kg (Peschka 1987). Part of the reason for this difference is that
the volume of hydrogen fuel is about four times greater than that
for the same energy content of gasoline. Although the hydrogen
storage vessel is large, hydrogen burns 1.33 times more
efficiently than gasoline in automobiles (Bockris and Wass 1988).
In tests, a BMW 745i liquid hydrogen test vehicle with a tank
weight of 75 kg, and the energy equivalent of 40 liters (320,000
kcal) of gasoline, had a cruising range in traffic of 400 km or a
fuel efficiency of 10 km per liter (24 mpg) (Winter 1986).

At present, commercial hydrogen is more expensive than
gasoline. For example, assuming 5˘ per kWh of electricity from a
conventional power plant, hydrogen would cost 9˘ per kWh
(Bockris and Wass 1988). This cost is the equivalent of
67˘/liter of gasoline. Gasoline sells at the pump in the United
States for approximately 30˘/liter. However, estimates are that
the real cost of burning a liter of gasoline ranges from $1.06 to
$1.32, when production, pollution, and other external costs are
included (Worldwatch Institute 1989). Therefore, based on these
calculations hydrogen fuel may eventually be competitive.

Some of the oxygen gas produced during the electrolysis of
water can be used to offset the cost of hydrogen. Also the oxygen
can be combined with hydrogen in a fuel cell, like those used in
the manned space flights. Hydrogen fuel cells used in rural and
suburban areas as electricity sources could help decentralize the
power grid, allowing central power facilities to decrease output,
save transmission costs, and make mass-produced, economical
energy available to industry.

Compared with ethanol, less land (0.15 ha versus 7 ha for
ethanol) is required for hydrogen production that uses
photovoltaics to produce the needed electricity. The
environmental impacts of hydrogen are minimal. The negative
impacts that occur during production are all associated with the
solar electric technology used in production.

Water for the production of hydrogen may be a problem in the
arid regions of the United States, but the amount required is
relatively small compared with the demand for irrigation water in
agriculture. Although hydrogen fuel produces emissions of
nitrogen oxides and hydrogen peroxide pollutants, the amounts are
about one-third lower than those produced from gasoline engines
(Veziroglu and Barbir 1992). Based on this comparative analysis,
hydrogen fuel may be a cost-effective alternative to gasoline,
especially if the environmental and subsidy costs of gasoline are
taken into account.

Hydroelectric systems

For centuries, water has been used to provide power for
various systems. Today hydropower is widely used to produce
electrical energy. In 1988 approximately 870 billion kWh (3 quads
or 9.5 % ) of the United States' electrical energy was produced
by hydroelectric plants (FERC 1988, USBC 1992a). Further
development and/or rehabilitation of existing dams could produce
an additional 48 billion kWh per year. However, most of the best
candidate sites already have been fully developed, although some
specialists project increasing US hydropower by as much as 100
billion kWh if additional sites are developed (USBC 1992a).

Hydroelectric plants require land for their water-storage
reservoirs. An analysis of 50 hydroelectric sites in the United
States indicated that an average of 75,000 ha of reservoir area
are required per 1 billion kWh/ yr produced (Table 2). However,
the size of reservoir per unit of electricity produced varies
widely, ranging from 482 ha to 763,000 ha per 1 billion kWh/yr
depending upon the hydro head, terrain, and additional uses made
of the reservoir (Table 2). The latter include flood control,
storage of water for public and irrigation supplies, and/or
recreation (FERC 1984). For the United States the energy
input/output ratio was calculated to be 1:48 (Table 2); for
Europe an estimate of 1:15 has been reported (Winter et al.
1992).

Based on regional estimates of land use and average annual
energy generation, approximately 63 million hectares of the total
of 917 million ha of land area in the United States are currently
covered with reservoirs. To develop the remaining best candidate
sites, assuming land requirements similar to those in past
developments, an additional 24 million hectares of land would be
needed for water storage (Table 3).

Reservoirs constructed for hydroelectric plants have the
potential to cause major environmental problems. First, the
impounded water frequently covers agriculturally productive,
alluvial bottomland. This water cover represents a major loss of
productive agricultural land. Dams may fail, resulting in loss of
life and destruction of property. Further, dams alter the
existing plant and animal species in the ecosystem (Flavin 1985).
For example, cold water fishes may be replaced by warm water
fishes, frequently blocking fish migration (Hall et al. 1986).
However, flow schedules can be altered to ameliorate many of
these impacts. Within the reservoirs, fluctuations of water
levels alter shorelines and cause downstream erosion and changes
in physiochemical factors, as well as the changes in aquatic
communities. Beyond the reservoirs, discharge patterns may
adversely reduce downstream water quality and biota, displace
people, and increase water evaporation losses (Barber 1993).
Because of widespread public environmental concerns, there
appears to be little potential for greatly expanding either large
or small hydroelectric power plants in the future (Table 3).

Wind power

For many centuries, wind power like water power has provided
energy to pump water and run mills and other machines. In rural
America windmills have been used to generate electricity since
the early 1900s.

Modern wind turbine technology has made significant advances
over the last 10 years. Today, small wind machines with 5 to 40
kW capacity can supply the normal electrical needs of homes and
small industries (Twidell 1987). Medium-size turbines rated 100
kW to 500 kW produce most of the commercially generated
electricity. At present, the larger, heavier blades required by
large turbines upset the desirable ratio between size and weight
and create efficiency problems. However, the effectiveness and
efficiency of the large wind machines are expected to be improved
through additional research and development of lighter weight but
stronger components (Clarke 1991). Assuming a 35% operation
capacity at a favorable site, the energy input/output ratio of
the system is 1:5 for the material used in the construction of
medium size wind machines (Table 2).

The availability of sites with sufficient wind (at least 20
km/in) limits the widespread development of wind farms.
Currently, 70% of the total wind energy (0.01 quad) produced in
the United States is generated in California (Table 3; AWEA
1992). However, an estimated 13% of the contiguous US land area
has wind speeds of 22 km/in or higher; this area then would be
sufficient to generate approximately 20 % of US electricity using
current technology (DOE 1992). Promising areas for wind
development include the Great Plains and coastal regions.

Another limitation of this energy resource is the number of
wind machines that a site can accommodate. For example, at
Altamont Pass, California, an average of one turbine per 1.8 ha
allows sufficient spacing to produce maximum power (Smith and
Ilyin 1991). Based on this figure approximately 11,700 ha of land
are needed to supply 1 billion kWh/ yr (Table 2). However,
because the turbines themselves only occupy approximately 2% of
the area or 230 ha, dual land use is possible. For example,
current agricultural land developed for wind power continues to
be used in cattle, vegetable, and nursery stock production.

An investigation of the environmental impacts of wind energy
production reveals a few hazards. For example, locating the wind
turbines in or near the flyways of migrating birds and wildlife
refuges may result in birds flying into the supporting structures
and rotating blades (Clarke 1991, Kellett 1990). Clarke suggests
that wind farms be located at least 300 meters from nature
reserves to reduce this risk to birds.

Insects striking turbine blades will probably have only a
minor impact on insect populations, except for some endangered
species. However, significant insect accumulation on the blades
may reduce turbine efficiency (Smith 1987a).

Wind turbines create interference with electromagnetic
transmission, and blade noise may be heard up to 1 km away
(Kellet 1990). Fortunately, noise and interference with radio and
television signals can be eliminated by appropriate blade
materials and careful placement of turbines. In addition, blade
noise is offset by locating a buffer zone between the turbines
and human settlements. New technologies and designs may minimize
turbine generator noise.

Under certain circumstances shadow flicker has caused
irritation, disorientation, and seizures in humans (Steele 1991).
However, as with other environmental impacts, mitigation is
usually possible through careful site selection away from homes
and offices. This problem slightly limits the land area suitable
for wind farms.

Although only a few wind farms supply power to utilities in
the United States, future widespread development may be
constrained because local people feel that wind farms diminish
the aesthetics of the area (Smith 1987a). Some communities have
even passed legislation to prevent wind turbines from being
installed in residential areas (Village of Cayuga Heights, New
York, Ordinance 1989). Likewise areas used for recreational
purposes, such as parks, limit the land available for wind power
development.

Photovoltaics

Photovoltaic cells are likely to provide the nation with a
significant portion of its future electrical energy (DeMeo et al
1991). Photovoltaic cells produce electricity when sunlight
excites electrons in the cells. Because the size of the units is
flexible and adaptable, photovoltaic cells are ideal for use in
homes, industries, and utilities.

Before widespread use, however, improvements are needed in the
photovoltaic cells to make them economically competitive. Test
photovoltaic cells that consist of silicon solar cells are
currently up to 21% efficient in converting sunlight into
electricity (Moore 1992). The durability of photovoltaic cells,
which is now approximately 20 years, needs to be lengthened and
current production costs reduced about fivefold to make them
economically feasible. With a major research investment, all of
these goals appear possible to achieve (DeMeo et al. 1991).

Currently, production of electricity from photovoltaic cells
costs approximately 30˘/kWh, but the price is projected to fall
to approximately 10˘/kWh by the end of the decade and perhaps
reach as low as 4˘ by the year 2030, provided the needed
improvements are made (Flavin and Lenssen 1991). In order to make
photovoltaic cells truly competitive, the target cost for modules
would have to be approximately 8˘/ kWh (DeMeo et al. 1991).

Using photovoltaic modules with an assumed 7.3% efficiency
(the current level of commercial units), 1 billion kWh/yr of
electricity could be produced on approximately 2700 ha of land
(Table 2), or approximately 0.027 ha per person, based on the
present average per capita use of electricity. Thus, total US
electrical needs theoretically could be met with photovoltaic
cells on 5.4 million ha (0.6% of US land). If 21% efficient cells
were used, the total area needed would be greatly reduced.
Photovoltaic plants with this level of efficiency are being
developed (DeMeo et al. 1991).

The energy input for the structural materials of a
photovoltaic system delivering 1 billion kWh is calculated to be
approximately 300 kWh/m2. The energy input/output ratio for
production is about 1:9 assuming a life of 20 years (Table 2).

Locating the photovoltaic cells on the roofs of homes,
industries, and other buildings would reduce the need for
additional land by approximately 5% (USBC 1992a), as well as
reduce the costs of energy transmission. However, photovoltaic
systems require back­up with conventional electrical systems,
because they function only during daylight hours.

The major environmental problem associated with photovoltaic
systems is the use of toxic chemicals such as cadmium sulfide and
gallium arsenide, in their manufacture (Holdren et al. 1980).
Because these chemicals are highly toxic and persist in the
environment for centuries, disposal of inoperative cells could
become a major environmental problem. However, the most promising
cells in terms of low cost, mass production, and relatively high
efficiency are those being manufactured using silicon. This
material makes the cells less expensive and environmentally safer
than the heavy metal cells.

Solar thermal conversion systems

Solar thermal energy systems collect the sun's radiant energy
and convert it into heat. This heat can be used for household and
industrial purposes and also to drive a turbine and produce
electricity. System complexity ranges from solar ponds to the
electric-generating central receivers. We have chosen to analyze
electricity in order to facilitate comparison to the other solar
energy technologies.

Solar ponds. Solar ponds are used to capture solar
radiation and store it at temperatures of nearly 100°C. Natural
or man-made ponds can be made into solar ponds by creating a
salt-concentration gradient made up of layers of increasing
concentrations of salt. These layers prevent natural convection
from occurring in the pond and enable heat collected from solar
radiation to be trapped in the bottom brine.

The hot brine from the bottom of the pond is piped out for
generating electricity. The steam from the hot brine turns freon
into a pressurized vapor, which drives a Rankine engine. This
engine was designed specifically for converting low-grade heat
into electricity. At present, solar ponds are being used in
Israel to generate electricity (Tabor and Doran 1990).

For successful operation, the salt concentration gradient and
the water levels must be maintained. For example, 4000 ha of
solar ponds lose approximately 3 billion liters of water per year
under the arid conditions of the southwestern United States
(Tabor and Doran 1990). In addition, to counteract the water loss
and the upward diffusion process of salt in the ponds, the dilute
salt water at the surface of the ponds has to be replaced with
fresh water. Likewise salt has to be added periodically to the
heat-storage zone. Evaporation ponds concentrate the brine, which
can then be used for salt replacement in the solar ponds.

Approximately 4000 ha of solar ponds (40 ponds of 100 ha) and
a set of evaporation ponds that cover a combined 1200 ha are
needed for the production of 1 billion kWh of electricity needed
by 100,000 people in one year (Table 2). Therefore, a family of
three would require approximately 0.2 ha (22,000 sq ft) of solar
ponds for its electricity needs. Although the required land area
is relatively large, solar ponds have the capacity to store heat
energy for days, thus eliminating the need for back-up energy
sources from conventional fossil plants. The efficiency of solar
ponds in converting solar radiation into heat is estimated to be
approximately 1:5. Assuming a 30-year life for a solar pond, the
energy input/output ratio is calculated to be 1:4 (Table 2). A
100 hectare (1 km2) solar pond is calculated to produce
electricity at a rate of approximately 14˘ per kWh. According to
Folchitto (1991), this cost could be reduced in the future.

In several locations in the United States solar ponds are now
being used successfully to generate heat directly. The heat
energy from the pond can be used to produce processed steam for
heating at a cost of only 2˘ to 3.5˘ per kWh (Gommend and
Grossman 1988). Solar ponds are most effectively employed in the
Southwest and Mid-west.

Some hazards are associated with solar ponds, but most can be
prevented with careful management. For instance, it is essential
to use plastic liners to make the ponds leakproof and thereby
prevent contamination of the adjacent soil and groundwater with
salt. Burrowing animals must be kept away from the ponds by
buried screening (Dickson and Yates 1983). In addition, the ponds
should be fenced to prevent people and other animals from coming
in contact with them. Because some toxic chemicals are used to
prevent algae growth on water surface and freon is used in the
Rankine engine, methods will have to be devised for safely
handling these chemicals (Dickson and Yates 1983).

Solar receiver systems. Other solar thermal
technologies that concentrate solar radiation for large scale
energy production include distributed and central receivers.
Distributed receiver technologies use rows of parabolic troughs
to focus sunlight on a central-pipe receiver that runs above the
troughs. Pressurized water and other fluids are heated in the
pipe and are used to generate steam to drive a turbogenerator for
electricity production or provide industry with heat energy.

Central receiver plants use computer-controlled, sun-tracking
mirrors, or heliostats, to collect and concentrate the sunlight
and redirect it toward a receiver located atop a centrally placed
tower. In the receiver, the solar energy is captured as heat
energy by circulating fluids, such as water or molten salts, that
are heated under pressure. These fluids either directly or
indirectly generate steam, which is then driven through a
conventional turbogenerator to yield electricity. The receiver
system may also be designed to generate heat for industry.

Distributed receivers have entered the commercial market
before central receivers, because central receivers are more
expensive to operate. But, compared to distributed receivers,
central receivers have the potential for greater efficiency in
electricity production because they are able to achieve higher
energy concentrations and higher turbine inlet temperatures
(Winter 1991). Central receivers are used in this analysis.

The land requirements for the central receiver technology are
approximately 1100 ha to produce 1 billion kWh/yr (Table 2),
assuming peak efficiency, and favorable sunlight conditions like
those in the western United States. Proposed systems offer four
to six hours of heat storage and may be constructed to include a
back-up alternate energy source. The energy input/output ratio is
calculated to be 1:10 (Table 2). Solar thermal receivers are
estimated to produce electricity at approximately 10˘ per kWh,
but this cost is expected to be reduced somewhat in the future,
making the technology more competitive (Vant-Hull 1992). New
technical advances aimed at reducing costs and improving
efficiency include designing stretched membrane heliostats,
volumetric-air ceramic receivers, and improved overall system
designs (Beninga et al. 1991).

Central receiver systems are being tested in Italy, France,
Spain, Japan, and the United States (at the 10-megawatt Solar One
pilot plant near Barstow, California; Skinrood and Skvarna 1986).
Also, Luz's Solar Electric Generating System plants at Barstow
use distributed receivers to generate almost 300 MW of commercial
electricity (Jensen et al. 1989).

The potential environmental impacts of solar thermal receivers
include: the accidental or emergency release of toxic chemicals
used in the heat transfer system (Baechler and Lee 1991); bird
collisions with a heliostat and incineration of both birds and
insects if they fly into the high temperature portion of the
beams; and--if one of the heliostats did not track properly but
focused its high temperature beam on humans, other animals, or
flammable materials--burns, retinal damage, and fires (Mihlmester
et al. 1980). Flashes of light coming from the heliostats may
pose hazards to air and ground traffic (Mihlmester et al. 1980).

Other potential environmental impacts include microclimate
alteration, for example reduced temperature and changes in wind
speed and evapotranspiration beneath the heliostats or collecting
troughs. This alteration may cause shifts in various plant and
animal populations. The albedo in solar­collecting fields may be
increased from 30% to 56% in desert regions (Mihlmester et al.
1980). An area of 1100 ha is affected by a plant producing 1
billion kWh.

Approximately 23% (18.4 quads) of the fossil energy consumed
yearly in the United States is used for space heating and cooling
of buildings and for heating hot water (DOE 1991a). At present
only 0.3 quads of energy are being saved by technologies that
employ passive and active solar heating and cooling of buildings
(Table 2). Tremendous potential exists for substantial energy
savings by increased energy efficiency and by using solar
technologies for buildings.

Both new and established homes can be fitted with solar
heating and cooling systems. Installing passive solar systems
into the design of a new home is generally cheaper than
retrofitting an existing home. Including passive solar systems
during new home construction usually adds less than 10% to
construction costs (Howard and Szoke 1992); a 3-5% added first
cost is typical.1 Based on the cost of construction and the
amount of energy saved measured in terms of reduced heating
costs, we estimate the cost of passive solar systems to be
approximately 3˘ per kWh saved.

Improvements in passive solar technology are making it more
effective and less expensive than in the past. In the area of
window designs, for example, current research is focused on the
development of superwindows with high insulating values and smart
or electrochromic windows that can respond to electrical current,
temperature, or incident sunlight to control the admission of
light energy (Warner 1991). Use of transparent insulation
materials makes window designs that transmit from 50% to 70% of
incident solar energy while at the same time providing insulating
values typical of 25 cm of fiber glass insulation (Chahroudi
1992). Such materials have a wide range of solar technology
applications beyond windows, including house heating with
transparent, insulated collector­storage walls and integrated
storage collectors for domestic hot water (Wittwer et al. 1991).

Active solar heating technologies are not likely to play a
major role in the heating of buildings. The cost of energy saved
is relatively high compared with passive systems and conservation
measures.2

Solar water heating is also cost effective. Approximately 3%
of all the energy used in the United States is for heating water
in homes (DOE 1991a). In addition, many different types of
passive and active water heating solar systems are available and
are in use throughout the United States. These systems are
becoming increasingly affordable and reliable (Wittwer et al.
1991). The cost of purchasing and installing an active solar
water heater ranges from $2500 to $6000 in the northern regions
and $2000 to $4000 in the southern regions of the nation (DOE/ CE
1988).

Although none of the passive heating and cooling technologies
require land, they can cause environmental problems. For example,
some indirect land-use problems may occur, such as the removal of
trees, shading, and rights to the sun (Schurr et al. 1979). Glare
from collectors and glazing could create hazards to automobile
drivers, pedestrians, bicyclists, and airline pilots. Also, when
houses are designed to be extremely energy efficient and
airtight, indoor air quality becomes a concern because air
pollutants may accumulate inside. However, installation of
well­designed ventilation systems promotes a healthful exchange
of air while reducing heat loss during the winter and heat gain
during the summer. If radon is a pollutant present at unsafe
levels in the home, various technologies can mitigate the problem
(ASTM 1992).

Comparing solar power to coal and nuclear
power

Coal and nuclear power production are included in this
analysis to compare conventional sources of electricity
generation to various future solar energy technologies. Coal,
oil, gas, nuclear, and other mined fuels are used to meet 92% of
US energy needs (Table 1). Coal and nuclear plants combined
produce three quarters of US electricity (USBC 1992a).

Energy efficiencies for both coal and nuclear fuels are low
due to the thermal law constraint of electric generator designs:
coal is approximately 35% efficient and nuclear fuels
approximately 33% (West and Kreith 1988). Both coal and nuclear
power plants in the future may require additional structural
materials to meet clean air and safety standards. However, the
energetic requirements of such modifications are estimated to be
small compared with the energy lost due to conversion
inefficiencies

The costs of producing electricity using coal and nuclear
energy are 3 ˘ and 5˘ per kWh, respectively (EIA 1990).
However, the costs of this kind of energy generation are
artificially low because they do not include such external costs
as damages from acid rain produced from coal and decommissioning
costs for the closing of nuclear plants. The Clean Air Act and
its amendments may raise coal generation costs, while the new
reactor designs, standardization, and streamlined regulations may
reduce nuclear generation costs. Government subsidies for nuclear
and coal plants also skew the comparison with solar energy
technologies (Wolfson 1991).

Clouding the economic costs of fossil energy use are the
direct and indirect US subsidies that hide the true cost of
energy and keep the costs low, thereby encouraging energy
consumption. The energy industry receives a direct subsidy of
$424 per household per year (based on an estimated maximum of $36
billion for total federal energy subsidies [ASK 1993]). In
addition, the mined-energy industry, like the gasoline industry,
does not pay for the environmental and public health costs of
fossil energy production and consumption.

The land requirements for fossil fuel and nuclear-based plants
are lower than those for solar energy technologies (Table 2). The
land area required for electrical production of 1 billion
kWh/year is estimated at 363 ha for coal and 48 ha for nuclear
fuels. These figures include the area for the plants and both
surface and underground mining operations and waste disposal. The
land requirements for coal technology are low because it uses
concentrated fuel sources rather than diffuse solar energy.
However, as the quality of fuel ore declines, land requirements
for mining will increase. In contrast, efficient reprocessing and
the use of nuclear breeder reactors may decrease the land area
necessary for nuclear power.

Many environmental problems are associated with both coal and
nuclear power generation (Pimentel et al. 1994). For coal, the
problems include the substantial damage to land by mining, air
pollution, acid rain, global warming, as well as the safe
disposal of large quantities of ash (Wolfson 1991). For nuclear
power, the environmental hazards consist mainly of radioactive
waste that may last for thousands of years, accidents, and the
decommissioning of old nuclear plants (Wolfson 1991).

Fossil-fuel electric utilities account for two-thirds of the
sulfur dioxide, one-third of the nitrogen dioxide, and one-third
of the carbon dioxide emissions in the United States (Kennedy et
al. 1991). Removal of carbon dioxide from coal plant emissions
could raise costs to 12˘/kWh; a disposal tax on carbon could
raise coal electricity costs to 18˘/kWh (Williams et al. 1990).

The occupational and public health risks of both coal and
nuclear plants are fairly high, due mainly to the hazards of
mining, ore transportation, and subsequent air pollution during
the production of electricity. However, there are 22 times as
many deaths per unit of energy related to coal than of nuclear
energy production because 90,000 times greater volume of coal
than nuclear ore is needed to generate an equivalent amount of
electricity.3

Also, and as important, coal produces more diffuse pollutants
than nuclear fuels during normal operation of the generating
plant. Coal fired plants produce air pollutants-- including
sulfur oxides, nitrogen oxides, carbon dioxide, and
particulates--that adversely affect air quality and contribute to
acid rain. Technologies do exist for removing most of the air
pollutants, but their use increases the cost of a new plant by
20-25% (IEA 1987). By comparison, nuclear power produces many
fewer pollutants than do coal plants (Tester et al. 1991).

Transition to solar energy and other
alternatives

The first priority of a sustainable US energy program should
be for individuals, communities, and industries to conserve
fossil energy resources. Other developed countries have proven
that high productivity and a high standard of living can be
achieved with considerably less energy expenditure compared to
that of the United States. Improved energy efficiency in the
United States, other developed nations, and even in developing
nations would help both extend the world's fossil energy
resources and improve the environment (Pimentel et al. 1994).

The supply and demand for fossil and solar energy; the
requirements of land for food, fiber, and lumber; and the rapidly
growing human population will influence future US options. The
growth rate of the US population has been increasing and is now
at 1.1 % per year (USBC 1992b); at this rate, the present
population of 260 million will increase to more than a half
billion in just 60 years. The presence of more people will
require more land for homes, businesses, and roads. Population
density directly influences food production, forest product
needs, and energy requirements. Considerably more agricultural
and forest land will be needed to provide vital food and forest
products, and the drain on all energy resources will increase.
Although there is no cropland shortage at present (USDA 1992),
problems undoubtedly will develop in the near future in response
to the diverse needs of the growing US population.

Solar energy technologies, most of which require land for
collection and production, will compete with agriculture and
forestry in the United States and worldwide (Table 2). Therefore,
the availability of land is projected to be a limiting factor in
the development of solar energy. In the light of this constraint,
an optimistic projection is that the current level of nearly 7
quads of solar energy collected and used annually in the United
States could be increased to approximately 37 quads (Ogden and
Williams 1989, Pimentel et al. 1984). This higher level
represents only 43% of the 86 quads of total energy currently
consumed in the United States (Tables 1 and 3). Producing 37
quads with solar technologies would require approximately 173
million ha, or nearly 20% of US land area (Table 3). At present
this amount of land is available, but it may become unavailable
due to future population growth and increased resource
consumption. If land continues to be available, the amounts of
solar energy (including hydropower and wind) that could be
produced by the year 2050 are projected to be: 5 quads from
biomass, 4 quads from hydropower, 8 quads from wind power, 6
quads from solar thermal systems, 6 quads from passive and active
solar heating, and 8 quads from photovoltaics (Table 3).

Another possible future energy source is fusion energy
(Bartlett 1994, Matare 1989). Fusion uses nuclear particles
called neutrons to generate heat in a fusion reactor vessel.
Nuclear fusion differs from fission in that the production of
energy does not depend on continued mining. However, high costs
and serious environmental problems are anticipated (Bartlett
1994). The environmental problems include the production of
enormous amounts of heat and radioactive material.

The United States could achieve a secure energy future and a
satisfactory standard of living for everyone if the human
population were to stabilize at an estimated optimum of 200
million (down from today's 260 million) and conservation measures
were to lower per capita energy consumption to about half the
present level (Pimentel et al. 1994). However, if the US
population doubles in 60 years as is more likely, supplies of
energy, food, land, and water will become inadequate, and land,
forest, and general environmental degradation will escalate
(Pimentel et al. 1994, USBC 1992a).

Fossil energy subsidies should be greatly diminished or
withdrawn and the savings should be invested to encourage the
development and use of solar energy technologies. This policy
would increase the rate of adoption of solar energy technologies
and lead to a smooth transition from a fossil fuel economy to one
based on solar energy. In addition, the nation that becomes a
leader in the development of solar energy technologies is likely
to capture the world market for this industry.

Conclusions

This assessment of alternate technologies confirms that solar
energy alternatives to fossil fuels have the potential to meet a
large portion of future US energy needs, provided that the United
States is committed to the development and implementation of
solar energy technologies and that energy conservation is
practiced. The implementation of solar technologies will also
reduce many of the current environmental problems associated with
fossil fuel production and use.

An immediate priority IS to speed the transition from reliance
on nonrenewable energy sources to reliance on renewable,
especially solar based, energy technologies. Various combinations
of solar technologies should be developed consistent with the
characteristics of different geographic regions, taking into
account the land and water available and regional energy needs.
Combined, biomass energy and hydroelectric energy in the United
States currently provide nearly 7 quads of solar energy, and
their output could be increased to provide up to 9 quads by the
year 2050. The remaining 28 quads of solar renewable energy
needed by 2050 is projected to be produced by wind power,
photovoltaics, solar thermal energy, and passive solar heating.
These technologies should be able to provide energy without
interfering with required food and forest production.

If the United States does not commit itself to the transition
from fossil to renewable energy during the next decade or two,
the economy and national security will be adversely affected.
Starting immediately, it is paramount that US residents must work
together to conserve energy, land, water, and biological
resources. To ensure a reasonable standard of living in the
future, there must be a fair balance between human population
density and energy, land, water, and biological resources.

American Society for Testing Materials (ASTM). 1992. Standard
guide for radon control options for the design and construction
of new low rise residential buildings. Pages 1117-1123 in Annual/
Book of American Society for Testing Materials. E1465-92. ASTM,
Philadelphia, PA.